Defence against disease

• Pathogens that can infect humans include viruses, bacteria, fungi, and protists
• Archaea are not known to cause any diseases in humans
• Pathogens cause disease through infection and can pass from diseased hosts to healthy organisms, leading to infectious diseases
• The skin and mucous membranes act as the primary defense against pathogens that cause infectious diseases
• The skin acts as both a physical and chemical barrier to pathogens
• Mucous membranes consist of the internal linings of the lungs, trachea, and gut, producing a lining of sticky mucous that can trap pathogens
• When a blood vessel is ruptured, the blood-clotting mechanism is activated to prevent further blood loss
• Clotting factors are released from platelets, leading to the conversion of fibrinogen to fibrin by thrombin and trapping of erythrocytes to form a clot
• The immune response includes the innate immune system and the adaptive immune system
• The innate immune system responds to broad categories of pathogens and does not change during an organism's life
• Phagocytes are components of the innate immune system
• The adaptive immune system responds specifically to pathogens and builds up memory of encountered pathogens for a more effective immune response
• Lymphocytes are responsible for the specific immune response and circulate in the blood and lymph nodes
• Phagocytes move from blood to sites of infection by ameboid movements and digest pathogens using enzymes from lysosomes
• Lymphocytes include B lymphocytes that are produced in the bone marrow and cooperate to produce antibodies
• Most antigens are glycoproteins or proteins located on the outer surfaces of pathogens
• Antigens on erythrocytes may stimulate antibody production if transfused into a person with a different blood group
• B cells produce antibodies and become memory cells when activated by direct interaction with a specific antigen and contact with a helper T cell
• Activated B cells multiply to form clones of antibody-secreting plasma cells
• Immunity is the ability to eliminate an infectious disease from the body due to the long-term survival of lymphocytes capable of making specific antibodies
• HIV infects certain types of lymphocytes, reducing the ability to produce antibodies and fight infections
• Antibiotics block processes in bacteria but not in eukaryotic cells
• Zoonoses are infectious diseases that can transfer from other species to humans, including examples like tuberculosis, rabies, and COVID-19
• Vaccines contain antigens or nucleic acids that stimulate immunity to specific pathogens without causing the disease
• Herd immunity occurs when a sufficient percentage of a population is immune to a disease, impeding transmission
• Two types of lymphocytes:

  • B lymphocytes:
  • Produced in the bone marrow
  • Mature in the bone marrow
  • Present in very large numbers
  • Produce specific antibodies
  • T lymphocytes:
  • Produced in the bone marrow
  • Mature in the thymus
  • Two types of T lymphocytes: Helper T lymphocytes and Cytotoxic T lymphocytes
• Lymphocytes in the adaptive immune system:

  • Y-shaped proteins
  • Made of 4 polypeptide chains: 2 heavy chains and 2 light chains
  • Include a constant region and a variable region
  • Antibodies attach to pathogens and prevent them from attaching to body cells
  • Some antibodies neutralize toxins produced by pathogens
  • Antibodies help phagocytes to recognize pathogens, allowing more of them to be engulfed
• Antigens:

  • Any molecule recognized as non-self
  • Most antigens are glycoproteins or other proteins located on the outer surfaces of pathogens
  • Each type of lymphocyte in the body recognizes only one specific antigen
• Blood types and antibodies:

  • Antibodies (immunoglobulins) are specific to antigens (agglutinogens)
  • Blood type O is the universal donor
  • Type AB is the universal recipient
• Activation of B lymphocytes by helper T lymphocytes:

  • Non-specific immune cells called macrophages engulf pathogens
  • Macrophages present antigenic fragments to specific TH lymphocytes
  • Specific B cells produce specific antibodies when stimulated by TH cells
  • Clones of plasma B cells are formed that produce the same type of antibody
  • Clones develop into short-lived plasma cells and long-lived memory cells
• Immunity:

  • Adaptive immune system relies on clonal expansion of plasma cells to produce antibodies
  • Memory cells are produced to prevent delay in antibody production upon re-exposure to the same antigen
  • Immunity eliminates infectious diseases from the body
• HIV effects:

  • Retrovirus that infects helper T cells, disabling the adaptive immune system
  • HIV specifically targets helper T lymphocytes
• Antibiotics:

  • Block processes in prokaryotic cells but not in eukaryotic cells
  • Target prokaryotic metabolism, including key enzymes, 70S ribosomes, and components of the cell wall
• Antibiotic resistance:

  • Genes confer resistance by degrading antibiotics, blocking entry, or increasing removal
  • Resistant strains of bacteria can proliferate rapidly and pass resistance genes to susceptible strains
• Zoonotic diseases:

  • Diseases transmitted from animals to humans are called zoonotic diseases
  • Examples include Tuberculosis, Rabies, Japanese encephalitis, and COVID-19
• Vaccines and immunization:

  • Vaccines contain antigens or nucleic acids incapable of causing disease
  • Vaccinations induce long-term immunity by stimulating the production of memory cells
  • Herd immunity occurs when a large portion of a community becomes immune to a disease